1. Field of the Invention
The present invention relates to a method of manufacturing a substrate for information storage media that has the necessary properties for future information storage media substrate purposes and, for example, to a method manufacturing a substrate for information storage media that deals with the increased density of future magnetic information storage media, is lower in specific gravity, high in Young's modulus, excels in fracture toughness, is extremely smooth in surface roughness after processing, excels in head sliding characteristics and impact resistance.
It should be noted that “information storage media” in the present invention indicates a magnetic information storage media that can be used in a hard disk for a variety of electronic devices.
2. Related Art
In recent years, for personal computers and various electronic devises, information storage devices of larger capacity have become necessary in order to handle large data files such as of video and sound. As a result, for magnetic information storage media, the demand for higher storage density has been raising year by year.
In order to handle this, the heat resistance of the substrate and smoothness of the surface in the next generation of magnetic storage systems have been demanded at levels that are high compared to existing substrates. In addition, possessing lowered specific gravity for reducing the load on the spindle motor, high mechanical strength for preventing crashing of the disk, and high fracture toughness to withstand impact with the head when the magnetic information storage fell down will become important still more than presently.
Furthermore, in recently years, since a part of the market of hard disks is being transferred to SSD (solid state drive), which are information storage devices employing flash memory, a much greater reduction in production cost has been demanded also in order to also appeal in unit cost per storage capacity, which is a significant over SSD.
For the materials used in substrates for information storage media, there are Al alloy substrates, glass substrates, crystallized glass substrates, etc.
Glass material substrates composed of glass and crystallized glass are superior in the aspect of the Vickers hardness being higher than Al alloy substrates, the aspect of having high surface smoothness, etc.; therefore, they are currently mostly used for purposes in which mobile use is assumed.
Glass material substrates are generally manufactured by the following method. Specifically, molten glass is formed by melting glass raw material, and this molten glass is formed into a plate shape. Herein, as a method of forming molten glass into a plate shape, there is a direct pressing method and a float method, which are methods of pressing molten glass. In the case of crystallized glass, crystals are caused to precipitate inside of the plate glass by heat treating. Next, a glass substrate or crystallized glass substrate is produced by processing the plate glass or crystallized glass into a disk shape, and carrying out a lapping process for bringing the plate thickness and flatness closer to the final shape, and a polishing process for obtaining a smooth surface texture.
Performing the lapping process using a loose grain method or fixed grain method is common practice conventionally. Herein, the loose grain method is performed by holding plate glass between upper and lower pressure plates, and moving relatively the pressure plates and plate glass by causing these to rotate, while supplying a slurry containing loose grains in a grinding fluid. In addition, the fixed grain method is performed by forming pellets from diamond particles using a bond such as resin, metal, and vitrified, and relatively moving the pressure plates and plate glass by causing these to rotate while supplying a grinding fluid (coolant) from the pressure plates in which these pellets are arranged in plurality.
The polishing process uses pressure plates to which polishing pads are adhered, and is performed by holding plate glass between the upper and lower pressure plates similarly to the lapping process, and relatively moving the pressure plates and plate glass by causing these to rotate while supplying a slurry containing loose grains composed of cerium oxide and the like.
The lapping process and the polishing process are performed separately in a plurality of stages and it is common for the abrasive to come to be smaller in each stage undergone.
If a substrate corresponding to a next generation of magnetic storage system were attempted to be produced with such a conventional manufacturing method, the following problem would arise. For example, in the case of a crystallized glass substrate, the type of crystalline phase precipitating to the glass phase, the grain size, and the amount of crystals largely affect the mechanical strength. Applying this fact, if the grain size and amount of crystals are controlled in order to raise the mechanical strength, and the precipitating crystalline phase is made to be crystals having high hardness, there have been problems in that the processing rates of lapping and polishing will deteriorate considerably or it will become difficult to obtain the smooth surface desired. The deterioration in processing rate is a primary factor greatly influencing an increase in manufacturing cost.
Cited Publication 1 discloses a substrate for information storage media composed of crystallized glass having a gahnite crystalline phase. If crystallized under the crystallization conditions disclosed in this publication, a substrate having a high degree of crystallinity will be obtained. Although this has high mechanical strength and high fracture toughness, the surface roughness thereof after polishing does not satisfy the level demanded for the next generation. In addition, the processing rates are low in the lapping process and polishing process due to the surface hardness being too high, and thus and an extended time period is required in the lapping process and polishing process. As a result, the yieldability of a substrate for information storage media is poor, and it is not possible to satisfy the demanded cost of the market.
Crystallized glass in which a spinel-type compound or a solid solution thereof is the main crystalline phase has previously been proposed as a material having high mechanical strength for application as information storage media substrate and structural material. However, it has been conventionally considered that having a high degree of crystallinity is necessary in order to have high mechanical strength. Therefore, the crystallized glass of gahnite type previously proposed has crystals that transmit almost no visible light precipitate, and has not obtained a surface texture to be applied as a substrate for a next generation of information storage media with a conventional processing method.
Cited Publication 2 discloses a substrate for information storage media composed of crystallized glass having an enstatite crystalline phase. Even if the crystallized glass disclosed in this publication is processed with a conventional processing method, due to the processing rate of the lapping process being low, the Ra of the surface texture is 0.2 nm to 0.3 nm after polishing; therefore, a smooth surface is not obtained.
Cited Publication 1: Japanese Unexamined Patent Application Publication No. H07-300340
Cited Publication 2: Japanese Unexamined Patent Application Publication No. 2004-220719